Peelable seal and method of making and using same

ABSTRACT

A peelable seal comprises a mixture of at least two immiscible polymers. The first polymer forms a continuous phase in the peelable seal, whereas the second polymer is dispersed in the continuous phase. For example, a peelable seal can also be made from a mixture of an ethylene polymer with a melt index in the range from about 0.1 to about 20 g/10 minutes and a propylene polymer with a melt flow rate in the range from about 0.01 to about 2 g/10 minutes. The two polymers define a shear viscosity differential: Δ=|(η 1 −η 2 )/η 1 |, wherein η 1  and η 2  are the respective shear viscosity for the first and second polymers at a temperature of 230° C. and a shear rate of about 100 radian/second. Preferably, the shear viscosity differential is less than about 100%.

PRIOR RELATED APPLICATIONS

[0001] This application claims priority to previously filed U.S.provisional patent application Serial No. 60/259,365, filed Jan. 2,2001, which is incorporated by referenced herein in its entirety.

FEDERALLY SPONSORED RESEARCH STATEMENT

[0002] Not applicable.

REFERENCE TO MICROFICHE APPENDIX

[0003] Not applicable.

FIELD OF THE INVENTION

[0004] The invention relates to a heat sealable and peelable film. Theinvention also relates to methods of making and using the heat sealable,peelable film.

BACKGROUND OF THE INVENTION

[0005] Heat sealable and peelable films (also referred to herein as“peelable seals”) are ubiquitously employed on a large scale fortemporarily closing containers that include, for example, food productsor medical devices. During use, a consumer tears away the peelable film.To gain consumer acceptance, a number of characteristics associated witha heat sealable and peelable film are desired. For example, the filmshould provide a leak-proof closure of the container or bag. To seal abag, heat sealing is commonly used. Various apparatus have beenconstructed for the purpose of forming bags while simultaneously fillingthe bags with the desired contents. These apparatus are commonly knownas vertical form-fill-and-seal and horizontal form-fill-and-sealmachines.

[0006] These machines typically have forming collars or bars that shapea flat piece of film into a tubular shape of a bag. Hot metal sealingjaws are moved from an open position to a closed position, contactingthe film in order to seal it into a bag shape. During the sealingprocess, the outer layer of the film comes into direct contact with thehot metal surface of the sealing jaws. Heat is thus transferred throughthe outer layer of the film to melt and fuse the inner sealant layer toform a seal. Generally, the outer layer has a higher melting temperaturethan the inner sealant layer. As such, while the inner sealant layer ismelted to form a seal, the outer layer of the film does not melt and isnot stuck to the sealing jaws. After the sealing jaws reopen, the filmis cooled to room temperature.

[0007] Before the inner sealant layer is cooled to room temperature, itshould be able to maintain its seal integrity. The ability of anadhesive or sealant layer to resist creep of the seal while it is stillin a warm or molten state is generally referred to as “hot tack.” Toform a good seal, the hot tack of the sealable and peelable film shouldbe adequate.

[0008] Besides adequate hot tack, it is also desirable to have a lowheat seal initiation temperature which helps to ensure fast packagingline speeds and a broad sealing window which could accommodatevariability in process conditions, such as pressure and temperature. Abroad sealing window also enables high speed packaging of heat sensitiveproducts, as well as, provides a degree of forgiveness for changes inpackaging or filling speeds.

[0009] In addition to the “sealable” characteristic of a sealable andpeelable film, it should also have a desired “peelable” characteristicneeded to provide an easily openable seal on a package or bag.Peelability generally refers to the ability to separate two materials orsubstrates in the course of opening a package without compromising theintegrity of either of the two. The force required to pull a seal apartis called “seal strength” or “heat seal strength” which can be measuredin accordance with ASTM F88-94. The desired seal strength variesaccording to specific end user applications. For flexible packagingapplications, such as cereal liners, snack food packages, cracker tubesand cake mix liners, the seal strength desired is generally in the rangeof about 1-9 pounds per inch. For example, for easy-open cereal boxliners, a seal strength in the range of about 2-3 pounds per inch iscommonly specified, although specific targets vary according toindividual manufactures requirements. In addition to flexible packagingapplication, a sealable and peelable film can also be used in rigidpackage applications, such as lidding for convenience items (e.g., snackfood such as puddings) and medical devices. Typical medical packageshave a seal strength of about 1-3 pounds per inch.

[0010] Additional desired characteristics for a heat sealable andpeelable film include a low coefficient of friction and good abuseresistance. A low coefficient of friction ensures that the sealant layercan be processed smoothly and efficiently on fabrication and packagingequipment and is particularly important for vertical form-fill-and-sealpackaging. Good abuse resistance and toughness is desired, for example,in cereal box liners to withstand tears and punctures fromirregularly-shaped, rigid cereals. Additional characteristics includetaste and odor performance and barrier or transmission properties.

[0011] Heat sealable and peelable films are generally made from one ormore polymeric resins. The resulting characteristics of a heat sealableand peelable film depend largely upon the type of the resins used toform the film. For example, ethylene vinyl acetate (EVA) and ethylenemethyl acrylate (EMA) copolymers provide excellent heat sealingproperties. However, the seals produced with these copolymers are suchthat separation usually cannot be achieved without damage to the film.To alleviate this problem, polybutylene is mixed with an EVA polymer toproduce a heat sealable and peelable film. Although the peelability ofthe film is improved, the heat sealable and peelable film has someunpleasant odor due to the presence of EVA. In addition to usingpolybutylene, some ionomers, such as SURLYN®, is mixed with EVA toproduce a heat sealable and peelable film. While the film is peelable,it causes stringiness or “angel hair” upon separation of the film.Moreover, ionomers are generally expensive and may have some odor aswell.

[0012] Although a number of resins systems have been employed to make aheat sealable and peelable film, there continues to exist a need for animproved heat sealable and peelable film with consistent seal strength.It is desirable that the resin system used to produce the heat sealableand peelable film has a relatively lower seal initiation temperature anda relatively broad heat sealing window. It is also desirable that theheat sealable and peelable film is relatively age-resistant and has arelatively lower coefficient of friction and good abuse resistance andtoughness.

SUMMARY OF THE INVENTION

[0013] The aforementioned needs are fulfilled by embodiments of theinvention in one or more of the following aspects. In one aspect, theinvention relates to a peelable seal which comprises a mixture of atleast two immiscible polymers; the first polymer forms a continuousphase and has a shear viscosity η₁ at a temperature of about 230° C. anda shear rate of about 100 radian/second; and the second polymer isdispersed in the continuous phase and has a shear viscosity η₂ at atemperature of about 230° C. and a shear rate of about 100radian/second. Moreover, the two polymers define a shear viscositydifferential: Δ=|(η₁−η₂)/η₁|, and the shear viscosity differential Δ isless than about 100%. The peelable seal may be a monolayer or amultilayer. In a multi-layered peelable seal, it may include at least abase layer and a skin layer which is formed from the mixture of the atleast two immiscible polymers. The peelable seal may also include twobase layers and a skin layer which is formed from the mixture of the atleast two immiscible polymers. In some embodiments, the shear viscositydifferential Δ may be less than about 50%, less than about 30%, lessthan about 20%, less than about 10%, less than about 5%. The shearviscosity differential Δ may also be zero or substantially close tozero.

[0014] In some embodiments, the first polymer is an ethylene polymer.The ethylene polymer may have a melt index in the range from about 0.1to about 20, preferably from 0.6 to about 10, and more preferably fromabout 1.5 to 3. In still other embodiments, the second polymer may be apropylene polymer. The propylene polymer may have a melt flow rate fromabout 0.01 to about 2, preferably from 0.1 to about 1, and morepreferably from about 0.3 to about 0.6. In other embodiments, the firstpolymer is an ethylene polymer with a melt index in the range from about1.5 to about 3, and the second polymer is a propylene polymer with amelt flow rate from about 0.3 to about 0.6.

[0015] In another aspect, the invention relates to a peelable seal whichcomprises a mixture of an ethylene polymer with a melt index in therange from about 0.1 to about 20 and a propylene polymer with a meltflow rate in the range from about 0.01 to about 2. Moreover, theethylene polymer forms a continuous phase of the peelable seal, and thepropylene polymer is dispersed in the continuous phase of the peelableseal. The peelable seal may be a monolayer or a multilayer. In amulti-layered peelable seal, it may include at least a base layer and askin layer formed from the mixture of the ethylene and the propylenepolymers. In some embodiments, the peelable seal may include two baselayers and a skin layer formed from the mixture of the ethylene polymerand the propylene polymer. The propylene polymer is uniformly dispersedin the continuous phase of the ethylene polymer. The ethylene polymermay have a melt index in the range from about 0.6 to about 10,preferably from about 1.5 to about 3. The ethylene polymer may have adensity from about 0.86 g/cc to about 0.97 g/cc, preferably from about0.86 g/cc to about 92 g/cc, and more preferably from about 0.88 g/cc toabout 0.92 g/cc. The ethylene polymer may be selected from high densitypolyethylene, low density polyethylene, linear low density polyethylene,very low density polyethylene, and ultra low density polyethylene. Theethylene polymer may be produced by a single site catalyst, ametallocene catalyst, a constrained geometry catalyst. The ethylenepolymer may be a substantially linear ethylene polymer or an ethylenepolymer with long chain branching. The propylene polymer may be ahomopolymer, a copolymer, or an interpolymer. The propylene polymer mayhave a melt flow rate in the range from about 0.01 to about 2,preferably from about 0.1 to about 1, and more preferably from about 0.3to about 0.5. In some embodiments, the ethylene polymer has a melt indexin the range from about 1.5 to about 3, and the propylene polymer has amelt flow rate of less than about 0.6. In other embodiments, theethylene polymer and the propylene polymer define a shear viscositydifferential: Δ=|(η_(e)−η_(p))/η_(e)|, in which η_(e) and η_(p) are therespective shear viscosity of the ethylene polymer and the propylenepolymer at a shear rate of about 100 radian/second and a temperature ofabout 230° C. Moreover, the shear viscosity differential Δ is than about100%. Preferably, the shear viscosity differential Δ is less than 50%,less than 25%, less than 10%, less than 5%, or zero or substantiallyzero.

[0016] In yet another aspect, the invention relates to a method ofmaking a peelable seal. As; The method includes: (a) obtaining aethylene polymer with a melt index in the range from about 0.1 to about20; (b) obtaining a propylene polymer with a melt flow rate in the rangefrom about 0.01 to about 2; (c) mixing the ethylene polymer and thepropylene polymer to obtain a blend therefrom, and (d) forming apeelable seal from the blend. Moreover, the ethylene polymer forms acontinuous phase in the peelable seal, and the propylene polymer forms adispersed phase in the peelable seal.

[0017] In still another aspect, the invention relates to a method ofmaking a peelable seal. The method includes: (a) obtaining a firstpolymer with a shear viscosity η₁ at a temperature of about 230° C. anda shear rate of about 100 radian/second; (b) obtaining a second polymerwith a shear viscosity η₂ at a temperature of about 230° C. and a shearrate of about 100 radian/second; (c) mixing the first polymer and thesecond polymer to form a blend; and (d) forming a peelable seal from theblend. Moreover, the first and second polymers define a shear viscositydifferential Δ=|(η₁−η₂)/η₁| less than 100%. The first polymer forms acontinuous phase in the peelable seal and the second polymer forms adispersed phase in the peelable seal.

[0018] In yet still another aspect, the invention relates to a polymerblend composition for a peelable seal. The polymer blend compositioncomprises a blend of an ethylene polymer with a melt index in the rangefrom about 0.1 to about 20 and a propylene polymer with a melt flow ratein the range from about 0.01 to about 2. The ethylene polymer is capableof forming a continuous phase of the peelable seal, and the propylenepolymer is capable of being dispersed in the continuous phase of thepeelable seal.

[0019] In one aspect, the invention relates to a polymer blendcomposition for a peelable seal. The polymer blend composition comprisesa blend of at least two immiscible polymers; a first polymer capable offorming a continuous phase and a second polymer capable of beingdispersed in the continuous phase. The first polymer and the secondpolymer have a shear viscosity η₁ and η₂ at a temperature of about 230°C. and a shear rate of 100 radian/second. Moreover, the two polymerdefine a shear viscosity differential: Δ=|(η₁−η₂)/η₁|, and the shearviscosity differential is less than about 100%.

[0020] Additional aspects of the invention and characteristics andadvantages provided by embodiments of the invention are apparent withthe following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a plot of viscosity data for an ethylene polymer and apropylene polymer used in embodiments of the invention: the solid linerepresents the shear viscosity of the ethylene polymer as a function ofshear rate 230° C., and the broken line represents the shear viscosityof the propylene polymer as a function of shear rate at 230° C.

[0022]FIG. 2 is a plot of viscosity data for the ethylene polymer ofFIG. 1 and another propylene polymer: the solid line represents theethylene polymer, and the broken line the propylene polymer.

[0023]FIG. 3 compares the peelable seal performance for various polymersblends: the triangles represent a peelable seal made from 100% of theethylene polymer of FIG. 1; the circles represent a peelable seal madefrom a blend of 65% of the ethylene polymer of FIG. 1 and 35% of thepropylene polymer of FIG. 1; and the diamonds represent a peelable sealmade from a blend of 65% of the ethylene polymer of FIG. 1 and 35% ofthe propylene polymer of FIG. 2.

[0024]FIG. 4 shows the age-resistance of the peelable seal made inaccordance with embodiments of the invention: the squares represent thepeelable seal tested after one day; and the diamonds represent thepeelable seal made from the same blend but tested after four weeks.

[0025]FIG. 5 is a plot showing the heat seal strength as a function ofseal bar temperature for various peelable seals made according toembodiments of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0026] In the following description, all numbers disclosed herein areapproximate values, regardless whether the word “about” or“approximately” is used in connection therewith. They may vary by up to1%, 2%, 5%, or sometimes 10 to 20%. Whenever a numerical range with alower limit, R_(L), and an upper limit R_(u), is disclosed, any number Rfalling within the range is specifically disclosed. In particular, thefollowing numbers R within the range are specifically disclosed:R=R_(L)+k*(R_(U)−R_(L)), wherein k is a variable ranging from 1% to 100%with a 1% increment, i.e. k is 1%, 2%, 3%, 4%, 5%, . . . , 50%, 51%,52%, . . . , 95%, 96%, 97% in or 100%. Moreover, any numerical rangedefined by two numbers, R, as defined in the above is also specificallydisclosed.

[0027] Embodiments of the invention provide a peelable seal and apolymer blend for making the peelable seal. The term “peelable seal”refers to an adhesive structure which may be attached to one or moresubstrates. The adhesive structure can be sealed by heat or othersealing methods to provide structural support. On the other hand, theseal structure can also be peeled apart when force is applied.Generally, a peelable seal is in the form of a film or layer. A peelableseal can be a monolayer or multilayer. For example, a peelable seal mayinclude two layers: one sealant layer and a base layer for support. Insome embodiments, a peelable seal may include three layers: a sealantlayer which is one of the outer layers and two base layers which may ormay not have the same compositions. Multiple layer structures, such as afour layered structure, five layered structure, six layered structure,or more layers, may also be made, if desired, so long as one of theouter layers is a heat sealant layer which is made from the polymerblend described herein.

[0028] The embodiments of the invention are based, in part, on therealization that a good peelable seal can be obtained from a blend oftwo or more polymers with certain desired rheological and morphologicalcharacteristics. Preferably, the sealant layer in the peelable sealshould include a continuous phase and at least one dispersed phase.Generally, the continuous phase and the dispersed phase are formed fromat least two immiscible polymers. Two polymers are “immiscible” whenthey do not form a homogenous mixture after being mixed. In other words,phase separation occurs in the mixture. One method to quantify theimmiscibility of two polymers is to use Hildbrand's solubility parameterwhich is a measure of the total forces holding the molecules of a solidor liquid together. Every polymer is characterized by a specific valueof solubility parameter, although it is not always available. Polymerswith similar solubility parameter values tend to be miscible. On theother hand, those with significantly different solubility parameterstend to be immiscible, although there are many exceptions to thisbehavior. Discussions of solubility parameter concepts are presented in(1) Encyclopedia of Polymer Science and Technology, Interscience, NewYork (1965), Vol. 3, pg. 833; (2) Encyclopedia of Chemical Technology,Interscience, New York (1971), Supp. Vol., pg. 889; and (3) PolymerHandbook, 3rd Ed., J. Brandup and E. H. Immergut (Eds.), (1989), JohnWiley & Sons “Solubility Parameter Values,” pp. VII-519, which areincorporated by referenced in their entirety herein.

[0029] The term “blend” herein refers to both dry blends and melt. Apolymer blend need not be uniform, although it is preferred. To obtainconsistent peel strength, the dispersed phase preferably should beuniformly distributed in the continuous phase. The dispersion qualitycan be affected by mixing conditions and equipment. Moreover, at a givenmixing pressure and speed, the viscosities of the components also affectthe dispersion quality. Sometimes, polymers with the same melt index mayhave significantly different viscosities in the high-shear stressenvironment of an extruder.

[0030] In embodiments of the invention, two immiscible polymers areselected such that their shear viscosities under normal processingconditions are not significantly different. For example, in the shearrate range between about 10 s⁻¹ to 1500 s⁻¹, the shear viscositiesbetween the two polymers preferably should not differ by more than 100%.In some embodiments, the difference in shear viscosity between the twopolymers are less than 50%, 30%, 20%, 10% or 5% in the aforementionedshear rate range.

[0031] One way to quantify the difference of shear viscosities betweentwo polymers is to define a shear viscosity differential:η=|(η₁−η₂)/η₁|, wherein η₁ and η₂ are the respective shear viscosity ata given temperature (e.g., 230° C.) and a given shear rate (e.g., 100s⁻¹) for the polymer forming the continuous phase (“the first polymer”)and the polymer forming the dispersed phase (“the second polymer”).Generally, Δ should be less than 100%. In some embodiments, Δ may be alower value, such as 50%, 30%, 20%, 10%, 5%, etc. It is also possible tohave a Δ which is zero or substantially close to zero.

[0032] Any polymer which has heat sealing property may be used inembodiments of the invention to form the continuous phase (i.e., thefirst polymer). Preferably, the polymer has a relatively low sealinitiation temperature and good hot tack strength. Suitable polymersinclude, but are not limited to, ethylene homopolymers and copolymers,ethylene/styrene copolymers, ethylene vinyl acetate (EVA) copolymers,ethylene methyl acrylates (EMA) copolymers, ethylene acrylic acidcopolymers, ethylene methacrylic acid copolymers (such as Nucrel®),hexene-butene copolymers, ionomers (such as Surlyn®), acid anhydridemodified ethylene vinyl acetate (such as Bynel®), and blends thereof.

[0033] Any polymer which is immiscible with the first polymer (i.e., thepolymer forming the continuous phase) may be used to form the dispersedphase (i.e., the second polymer). Suitable polymers, but are not limitedto, polybutylene, polypropylene homopolymers and copolymers, HDPE,crosslinked PE, terpolymers and blends thereof.

[0034] The polymer blend which forms a peelable seal in embodiments ofthe invention may include any amount of immiscible polymers. Generally,the first polymer may range from about 5% to 95% by weight. Preferably,it is present in the peelable seal from about 50% to 85%. On the otherhand, the second polymer may be present from about 5% to 95% by weight.Preferably, it is present from about 10% to about 50%. More preferably,it is present from about 20% to 40% by weight.

[0035] In some embodiments, peelable seals are formed from a polymerblend that includes an ethylene polymer and a propylene polymer. Theethylene polymer forms the continuous phase in the peelable seal,whereas the propylene polymer is dispersed in the continuous phase ofthe peelable seal. The ethylene polymer may have a melt index in therange from about 0.1 to 20 g/10 min., preferably from about 0.6 to 10g/10 min., and more preferably from about 1.5 to about 3 g/10 min. Thepropylene polymer may have a melt flow rate in the range from about 0.01to about 2 g/10 min., preferably from about 0.1 to 1 g/10 min., and mostpreferably from about 0.3 to about 0.5 g/10 min.. In other embodiments,a propylene polymer with a melt flow rate of less than 0.5 g/10 min. isblended with an ethylene polymer with a melt index in the range fromabout 1.5 to about 3 g/10 min.

[0036] A peelable seal obtained from the ethylene polymer/propylenepolymer blend as described above has one or more of the followingcharacteristics. First, the peelable seal is substantially odor-free,which is an improvement over a peelable seal made from ethylene vinylacetate (EVA) copolymers or ionomers. Moreover, the peelable seal givesa relatively clean peel. In other words, upon peeling, the peelable sealdoes not produce excessive stringiness (i.e., no substantial amount ofangel hair). As will be demonstrated below, the ethylenepolymer/propylene polymer blend produces a peelable seal which has aconsistent peel strength over a relatively wide temperature window. Suchblends also have good hot tack properties.

[0037] The following is a description of suitable ethylene polymers andpropylene polymers that may be used in embodiments of the invention.

ETHYLENE POLYMERS

[0038] An ethylene polymer is any polymer comprising greater than fiftymole percent of —CH₂CH₂— repeating units as derived from an ethylenemonomer or comonomer. Suitable ethylene polymers for use in embodimentsof the invention include any ethylene-containing polymers, bothhomopolymers and copolymers. Examples of ethylene polymers include, butare not limited to, ethylene homopolymers and ethylene interpolymers,such as low density polyethylene (LDPE), heterogeneously branchedethylene/α-olefin interpolymer (i.e., linear low density polyethylene(LLDPE), ultra low density polyethylene (ULDPE)), substantially linearethylene polymers (SLEP), and homogeneously branched ethylene polymer.

[0039] In some embodiments, the ethylene polymers are homogeneouslybranched (“homogeneous”) ethylene polymers, such as homogeneouslybranched linear ethylene/α-olefin interpolymers as described by Elstonin U.S. Pat. No. 3,645,992 or homogeneously branched substantiallylinear ethylene polymers as described by Lai et al. in U.S. Pat. Nos.5,272,236, 5,278,272, 5,665,800 and 5,783,638, the disclosures of whichare incorporated herein by reference. Homogeneously branched polymersare ethylene interpolymers in which the comonomer is randomlydistributed within a given interpolymer molecule and substantially allof the interpolymer molecules have the same ethylene/comonomer ratio,whereas heterogeneous interpolymers are those in which the interpolymermolecules do not have the same ethylene/comonomer ratio.

[0040] Homogeneous interpolymers can also be characterized by theirSCBDI (Short Chain Branch Distribution Index) or CDBI (CompositionDistribution Branch Index). The SCBDI or CBDI is defined as the weightpercent of the polymer molecules having a comonomer content within 50percent of the median total molar comonomer content. The CDBI of apolymer is readily calculated from data obtained from techniques knownin the art, such as, for example, temperature rising elutionfractionation (abbreviated herein as “TREF”) as described, for example,in Wild et al, Journal of Polymer Science, Poly. Phys. Ed., Vol. 20, p.441 (1982), in U.S. Pat. No. 4,798,081 (Hazlitt et al.), or U.S. Pat.No. 5,089,321 (Chum et al.), all disclosures of which are incorporatedherein by reference. Homogeneously branched linear ethyleneinterpolymers have a homogeneous (or narrow) short branchingdistribution (i.e., the polymer has a relatively high SCBDI or CDBI) butdoes not have long chain branching. That is, the ethylene interpolymerhas an absence of long chain branching and a linear polymer backbone inthe conventional sense of the term “linear.” The SCBDI or CDBI for thehomogeneous interpolymers and copolymers is preferably greater thanabout 50 percent, more preferably equal to or greater than about 70percent. Homogeneous interpolymers and polymers generally have a degreeof branching less than or equal to 2 methyls/1000 carbons in about 15percent (by weight) or less, preferably less than about 10 percent (byweight), and especially less than about 5 percent (by weight).

[0041] In some embodiments, substantially linear ethylene polymers withlong-chain branching are used. The term “substantially linear ethylenepolymer” as used herein means that the bulk ethylene polymer issubstituted, on average, with about 0.01 long chain branches/1000 totalcarbons to about 3 long chain branches/1000 total carbons (wherein“total carbons” includes both backbone and branch carbons). Somesubstantially linear ethylene polymers are substituted with about 0.01long chain branches/1000 total carbons to about 1 long chainbranches/1000 total carbons, preferably from about 0.05 long chainbranches/1000 total carbons to about 1 long chain branched/1000 totalcarbons, and more preferably from about 0.3 long chain branches/1000total carbons to about 1 long chain branches/1000 total carbons.

[0042] Long chain branching (LCB) may be defined herein as a chainlength of at least about 6 carbons, above which the length cannot bedistinguished by using ¹³C nuclear magnetic resonance spectroscopy.Alternatively, LCB may be defined as a chain length of at least one (1)carbon less than the number of carbons in the comonomer. For example, anethylene/1-octene polymer may have backbones with long chain branches ofat least seven (7) carbons in length, but it also may have short chainbranches of only six (6) carbons in length. Sometimes, a long chainbranch can be as long as the polymer backbone.

[0043] Long chain branching can be distinguished from short chainbranching by using ¹³C nuclear magnetic resonance (NMR) spectroscopy andto a limited extent, e.g. for ethylene homopolymers, it can bequantified using the method of Randall, (Rev. Macromol.Chem. Phys., C29(2&3), p. 285-297), the disclosure of which is incorporated herein byreference. However as a practical matter, current ¹³C nuclear magneticresonance spectroscopy cannot determine the length of a long chainbranch in excess of about six (6) carbon atoms and as such, thisanalytical technique cannot distinguish between a seven (7) carbonbranch and a seventy (70) carbon branch. As noted previously, long chainbranches can be as long as a polymer backbone.

[0044] Although conventional ¹³C nuclear magnetic resonance spectroscopycannot determine the length of a long chain branch in excess of sixcarbon atoms, there are other known techniques useful for quantifying ordetermining the presence of long chain branches in ethylene polymers,including ethylene/1-octene interpolymers. For example, U.S. Pat. No.4,500,648, incorporated herein by reference, teaches that long chainbranching frequency (LCB) can be represented by the equation LCB=b/M_(W)wherein b is the weight average number of long chain branches permolecule and M_(W) is the weight average molecular weight. The molecularweight averages and the long chain branching characteristics aredetermined by gel permeation chromatography and intrinsic viscositymethods, respectively.

[0045] For substantially linear ethylene polymers, the long chain branchis longer than the short chain branch that results from theincorporation of the α-olefin(s) into the polymer backbone. Theempirical effect of the presence of long chain branching in thesubstantially linear ethylene polymers used in the invention ismanifested as enhanced Theological properties which are quantified andexpressed herein in terms of gas extrusion rheometry (GER) resultsand/or melt flow, I₁₀/I₂, increases.

[0046] The substantially linear ethylene polymers used in embodiments ofthe invention are disclosed in the following U.S. Pat. Nos. 5,272,236;5,278,272; 5,783,638; and 6,060,567. The disclosure of all of thepreceding patents are incorporated by reference herein in theirentirety.

[0047] Metallocene single site polymerization catalysts (for example,the monocyclo-pentadienyl transition metal olefin polymerizationcatalysts described by Canich in U.S. Pat. No. 5,026,798 or by Canich inU.S. Pat. No. 5,055,438) or constrained geometry catalysts (for example,as described by Stevens et al. in U.S. Pat. No. 5,064,802) can be usedto manufacture substantially linear ethylene polymers in a mannerconsistent with the methods described in U.S. Pat. No. 5,272,236 and inU.S. Pat. No. 5,278,272. Additional polymerization methods are alsodescribed in PCT/US 92/08812 (filed Oct. 15, 1992). Preferably, thesubstantially linear ethylene polymers are manufactured using suitableconstrained geometry catalysts, especially constrained geometrycatalysts as disclosed in U.S. application Ser. Nos.: 545,403, filedJul. 3, 1990; U.S. Pat. Nos. 5,132,380; 5,064,802; 5,153,157; 5,470,993;5,453,410; 5,374,696; 5,532,394; 5,494,874; 5,189,192; the disclosuresof all of which are incorporated herein by reference in their entirety.Both metallocene and constrained geometry catalysts may be referred toas single-site catalysts in the art.

[0048] The substantially linear ethylene polymers used in embodiments ofthe invention are interpolymers of ethylene with at least one C₃-C₂₀α-olefin and/or C₄-C₁₈ diolefin. Copolymers of ethylene and an α-olefinof C₃-C₂₀ carbon atoms are especially preferred. The term “interpolymer”is used herein to indicate a copolymer, or a terpolymer, or the like,where at least one other comonomer is polymerized with ethylene to makethe interpolymer.

[0049] Suitable unsaturated comonomers useful for polymerizing withethylene include, for example, ethylenically unsaturated monomers,conjugated or non-conjugated dienes, polyenes, etc. Examples of suchcomonomers include C₃-C₂₀ α-olefins such as propylene, isobutylene,1-butene, 1-hexene, 1-pentene, 4-methyl-1-pentene, 1-heptene, 1-octene,1-nonene, 1-decene, and the like. Preferred comonomers includepropylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene,and 1-octene, and 1-heptene and 1-octene are especially preferred and1-octene is most especially preferred.

[0050] Other suitable monomers include styrene, halo- oralkyl-substituted styrenes, tetrafluoroethylene, vinylbenzocyclobutane,1,4-hexadiene, 1,7-octadiene, and cycloalkenes, e.g., cyclopentene,cyclohexene and cyclooctene.

[0051] The substantially linear ethylene polymers typically arecharacterized by a single melting peak as determined using differentialscanning calorimetry (DSC). However, the single melt peak may show,depending on equipment sensitivity, a “shoulder” or a “hump” on the sidelower of the melting peak (i.e. below the melting point) thatconstitutes less than 12 percent, typically less than 9 percent, moretypically less than 6 percent , of the total heat of fusion of thepolymer. This shoulder generally occurs within 34° C., typically within27° C., and more typically within 20° C. of the melting point of thesingle melting peak.

[0052] The single melting peak is determined by using a differentialscanning calorimeter standardized with indium and deionized water. Themethod involves about 5-7 mg sample sizes, a “first heat” to about 150°C. which is held for 4 minutes, a cool down at 10° C./min. to −30° C.which is held for 3 minutes, and heat up at 10° C./min. to 150° C. toprovide a “second heat” heat flow vs. temperature curve. Total heat offusion of the polymer is calculated from the area under the curve. Theheat of fusion attributable to a shoulder or hump artifact, if present,can be determined using an analytical balance and weight-percentcalculations.

[0053] The molecular weight distributions of ethylene polymers aredetermined by gel permeation chromatography (GPC) on a Waters 150° C.high temperature chromatographic unit equipped with a differentialrefractometer and three columns of mixed porosity. The columns aresupplied by Polymer Laboratories and are commonly packed with pore sizesof 10³, 10⁴, 10⁵ and 10⁶ Å. The solvent is 1,2,4-trichlorobenzene, fromwhich about 0.3 percent by weight solutions of the samples are preparedfor injection. The flow rate is about 1.0 milliliters/minute, unitoperating temperature is about 140° C. and the injection size is about100 microliters.

[0054] The molecular weight determination with respect to the polymerbackbone is deduced by using narrow molecular weight distributionpolystyrene standards (from Polymer Laboratories) in conjunction withtheir elution volumes. The equivalent polyethylene molecular weights aredetermined by using appropriate Mark-Houwink coefficients forpolyethylene and polystyrene (as described by Williams and Ward inJournal of Polymer Science, Polymer Letters, Vol. 6, p. 621, 1968, thedisclosure of which is incorporated herein by reference) to derive thefollowing equation:

M _(polyethylene) =a*(M _(polystyrene))^(b).

[0055] In this equation, a=0.4316 and b=1.0. Weight average molecularweight, M_(W), is calculated in the usual manner according to thefollowing formula: M_(j)=(Σw_(i)(M_(i) ^(j)))^(j); where w_(i) is theweight fraction of the molecules with molecular weight M_(i) elutingfrom the GPC column in fraction i and j=1 when calculating M_(w) andj=−1 when calculating M_(n).

[0056] For the ethylene polymers used in embodiments of the invention,the M_(w)/M_(n) is preferably less than 3.5, more preferably less than3.0, most preferably less than 2.5, and especially in the range of fromabout 1.5 to about 2.5 and most especially in the range from about 1.8to about 2.3.

[0057] The density of the ethylene polymers suitable for use inembodiments of the invention is generally less than 0.93grams/centimeter (g/cc), more preferably in the range from about 0.86g/cc to about 0.92 g/cc, and most preferably in the range from about0.88 g/cc to about 0.9 g/cc, as measured in accordance with ASTM D-792.

[0058] The molecular weight of the ethylene polymers can be convenientlydetermined using a melt index measurement according to ASTM D-1238,Condition 190° C./2.16 kg (formerly known as “Condition E” and alsoknown as I₂). Melt index is inversely proportional to the molecularweight of the polymer. Thus, the higher the molecular weight, the lowerthe melt index, although the relationship is not linear.

[0059] Preferably, the I₂ melt index of the ethylene polymers are in therange of from about 0.01 to about 50 g/10 minutes, more preferably fromabout 0.1 to about 20 g/10 minutes, and most preferably from about 0.4and about 12 g/10 minutes.

[0060] Other measurements useful in characterizing the molecular weightof ethylene polymer compositions involve melt index determinations withhigher weights, such as, ASTM D-1238, Condition 190° C./10 kg (formerlyknown as “Condition N” and also known as I₁₀). The ratio of a higherweight melt index determination to a lower weight determination is knownas a melt flow ratio, and for measured I₁₀ and the I₂ melt index valuesthe melt flow ratio is designated as I_(10/I) ₂. Preferably, theethylene polymers have an I₁₀/I₂ melt flow ratio greater than or equalto 6.8, more preferably greater than or equal to 8, and most preferablyin the range of from about 8.5 to about 20 and especially in the rangeof about 9 to about 15.

Propylene Polymers

[0061] Propylene polymers suitable for use in embodiments of theinvention may be either homopolymers or copolymers (random or impact).Propylene may be copolymerized with one or more monomers, such as anolefin. Suitable olefins include, but are not limited to, ethylene andalpha olefins, which include propylene, 1-butene, 1-pentene, 1-hexene,1-octene, 1-nonene, 1-decene, 1-unidecene, 1-dodecene,4-methyl-1-pentene, 4-methyl-1-hexene, 5-methyl-1-hexene,vinylcyclohexene, styrene, and the like. Preferred olefins and alphaolefins for copolymerization with propylene include ethylene, 1-butene,and other higher alpha olefins with at least 3 to about 20 carbon atoms,more preferably ethylene, butylene, and higher alpha olefins, mostpreferably ethylene. The comonomers or combination of comonomers, areused in any relative quantities within the definitions of the polymers.For propylene polymers, the comonomer content is preferably less thanabout 35%, more preferably between about 2-30%, and most preferablybetween about 5-20% by weight.

[0062] The propylene polymers may be atactic, syndiotactic, orisotactic. Isotacticity can be measured by C¹³ NMR. In some embodiments,the isotacticity of a propylene polymer is at least about 50 percent.

[0063] The propylene polymers are characterized by a melt flow rate,which is measured by ASTM D1238L at 230° C./2.16 kg. The melt flow rateof the propylene polymers is preferably between about 0.01 to about 2g/10 minutes, more preferably between about 0.1 to about 1 g/10 minutes,and most preferably between about 0.3 to about 0.5 g/10 minutes.

[0064] Suitable propylene polymers may have any molecular weightdistribution (MWD). MWD is calculated as the ratio M_(w)/M_(n), whereM_(w) is the weight average molecular weight and M_(n) is the numberaverage molecular weight. Preferably, the MWD of a propylene polymer isbetween about 1.5 to about 8, more preferably between about 2 to about5. Polymers with a MWD less than about 3 are generally made by using ametallocene or constrained geometry catalyst or using electron donorcompounds with Ziegler Natta catalysts.

[0065] In addition to propylene polymers described above, otherpropylene polymers described in the following U.S. Pat. Nos. may also beused in embodiments of the invention: 6,037,417; 5,902,848; 5,834,541;5,763,532; 5,731,362; 5,723,560; 5,596,052; 5,554,668; 5,541,236,5,538,804; 5,340,917; 5,001,197; 4,407,998; and 4,087,485. Thedisclosures of the all of the preceding patents are incorporated byreference herein in their entirety.

Peelable Seals

[0066] The peelable seals in accordance with embodiments of theinvention may be made by any method, including but not limited to,lamination and co-extrusion techniques or combinations thereof. Forexample, a peelable seal may be made by blown film techniques, cast filmtechniques, extrusion coating, injection molding, blow molding,transforming, profile extrusion, protrusion, compression molding,rotomolding, injection blow molding, and combination thereof. Simplyblown bubble film processes are described, for example, in Encyclopediaof Chemical Technology, Kirk-Othmer 3rd Ed., John Wiley & Sons, NewYork, (1981), Vol. 16, pp. 416-417 and Vol. 18, pp. 191-192, thedisclosures of which are incorporated by reference in their entiretyherein. Processes for manufacturing bi-axially oriented film, such asthe “double bubble” process, are described in the following U.S. Pat.Nos.: 3,456,044; 4,865,902; 4,352,849; 4,820,557; 4,927,708; 4,963,419;and 4,952,451. PCT Application WO 97/28960 and U.S. Pat. No. 4,665,130disclose various methods for making a peelable seal which can be used inembodiments of the invention. The disclosures of the PCT application andthe U.S. patent are incorporated by reference in their entirety herein.

[0067] As mentioned above, the peelable seal may be a monolayer film ora multi-layered film. A multi-layered film includes two or more layers.The first layer is a base layer or a barrier layer which comprises agas, aroma and/or moisture barrier material, such as high densitypolyethylene, medium density polyethylene, low density polyethylene,linear low density polyethylene, polypropylene, nylon, ethylene vinylalcohol copolymers, polyester, polyacrylonitrile, polyvinylidenechloride, and blends thereof. Generally, the base layer is made of amaterial that has a higher melting point than the sealant layer.Preferably, high density polyethylene is used as the base layer. In someembodiments, two high density polyethylene layers are co-extruded with asealant layer, which is formed from a polymer blend in accordance withembodiments of the invention. In addition to a base or barrier layer,additional layers may be provided to the peelable seal to obtain thedesired property, such as puncture resistance, tear resistance, opacitylevel, etc. It is also possible to include a color component (such astitanium oxide) in one or more of the base layers. Monolayer andmultilayer peelable seals may be made according to the film structuresand fabrication methods described in U.S. Pat. No. 5,685,128, which isincorporated by reference herein in its entirety.

[0068] The thickness of the peelable seal may range from about 0.3 milto about 20 mil, preferably from about 0.4 to 12 mil, more preferablyfrom about 1 mil to about 3 mil. The sealant layer should preferablycomprise about 3 to 80 percent of the total thickness of the peelableseal. In embodiments where there are two or more sealant layers, it ispreferred that each sealant layer ranges from about 3% to about 40%based upon the total thickness of the peelable seal. In embodimentswhere the peelable seal is formed by a blown film method, substantiallythe same thickness is obtained for each layer of the peelable seal.

[0069] The peelable seal in accordance with embodiments of the inventionhas many useful applications. For example, it can be used in packagesformed via form/fill/seal machinery, cook-in food packages, compressionfilled packages, heat sealable stretch wrap packaging films (such asfresh produce packaging and fresh red meat packaging). It also may beused in cereal box liners, cake mix packages, cracker tubes, medicaldevices, and item packaging, snack food convenience packages gaskets,etc.

[0070] The following examples are given to illustrate variousembodiments of the invention described herein. They should not beconstrued to limit the invention otherwise as described and claimedherein. All numerical values are approximate.

EXAMPLE 1

[0071] In this example, the shear viscosity data for an ethylene polymerand a propylene polymer were obtained. The ethylene polymer chosen inthis example was an ethylene/octene copolymer designated hereinafter as“PE1.” An example of PE1 resin is available from The Dow ChemicalCompany under the trademark of AFFINITY® PF1140, which is a homogeneoussubstantially linear ethylene interpolymer. This ethylene polymer waschosen due to its good seal performance, optical properties, abuseresistance, and low off-taste and off-odor. It also has good hot tackstrength. It is produced by the Dow constrained geometry catalyst asdisclosed in U.S. Pat. Nos.: 5,272,236; 5,380,810; and 5,783,638. Thesepatents also describe the characteristics of such polymers. Thedisclosures of all of the preceding patents are incorporated byreference in their entirety herein. Table 1 lists some physicalproperties of PE1 resin. TABLE 1 Physical Properties of PE1 ResinPhysical Properties Test Method Values Resin Properties Melt Index, g/10min ASTM D 1238 1.60 Density, g/cc ASTM D 792 0.8965 DSC Melting Point,° F. (° C.) Dow Method 205 (96) Vicat Softening Point, ° F. (° C.) ASTMD 1525 170 (77) Film Properties, 2.0 mil (51 μm) Puncture Resistance,ft-Ibf-in.³(J/cm³) Dow Method 245 (2) Energy, in.-Ibf (J) 72.4 (8)Force, Ibf (N) 18.8 (83) Dart Impart (Method B), g ASTM D 1709 >850Elmendorf Tear⁽¹⁾, g MD ASTM D 1922 470 CD 620 Tensile Yield, psi (MPa)MD ASTM D 882 840 (5.8) CD 920 (6.3) Ultimate Tensile, psi (MPa) MD ASTMD 882 7290 (50) CD 5730 (40) Ultimate Elongation, % MD ASTM D 882 690 CD700 Tensile Modulus, 2% Secant, MD ASTM D 882 10560 (73) psi (MPa) CD10610 (73) Clarity ASTM D 1746 70 Gloss, 20° ASTM D 2457 134 Haze, %ASTM D 1003 1.3 Seal Initiation Temperature⁽²⁾⁽³⁾, Dow Method 178 (81) °F. (° C.) Ultimate Seal Strength (Ib/in) 6.1 Hot tack Strength (N/in)7.5 ??? MWD ???

[0072] The propylene polymer selected for the viscosity measurement wasa propylene homopolymer designated herein as “PP1.” An example of PP1resin is available from The Dow Chemical Company as PP II103-00polypropylene. Another propylene homopolymer designated hereinafter as“PP2” which is comparable to PP1 is available from Montell under thetradename of Pro-fax™ 6823 polypropylene. Table 2 and Table 3 list somephysical properties of PP1 resin and PP2 resin, respectively. TABLE 2Physical Properties of PP1 Resin English Physical Properties Value UnitsTest Method Melt Flow Rate, 0.5 g/10 min ASTM D 1238 230° C./2.16 kgDensity (g/cc) 0.90 g/cm³ ASTM D 792B Tensile Strength at Yield 5200 psiASTM D 638 Elongation at Yield 13.0 % ASTM D 638 Flexural Modulus ASTM D790A (0.0t in/min., 1% secant) 230,000 psi Deflection Temperature 201 °F. ASTM D 648 Under Load @ 66 psi (0.45 Mpa), unannealed Notched Izod @73° F. 3.0 ft-lb/in ASTM D 256A (23° C.

[0073] TABLE 3 Physical Properties of PP2 Resin Nominal Values (English)Test Method Physical Properties Density-Specific Gravity 0.9000 sp grASTM D 792 23/23° C. Melt Flow Rate (230° C./2.16 kg-L) 0.500 g/10 min.ASTM D 1238 Water Absorption @ 24 hrs. 0.030% ASTM D 570 WaterAbsorption @ Sat. 0.200% ASTM D 570 Mechanical Tensile Strength @ Yield4800 psi ASTM D 638 Tensile Elongation @ Yield 13% ASTM D 638 FlexuralModulus, 1% Secant 180000 psi ASTM D 790² Impact Notched Izod Impact(73° F.) 1.50 ft-lb/in ASTM D 256 Hardness Rockwell Hardness (R-Scale)86.0 ASTM D 785 Thermal DTUL @ 66 psi-Unannealed 205° F. ASTM D 648 Max.Continuous Use Tmp 220° F. ASTM D 794 Melting Point 334° F. IgnitionCharacteristics Flame Rating - UL (0.0580 in., NC) HB UL 94 UL 746 RelTemp Indx Mech w/olmp 230° F. UL 746 (0.0580 in) Rel Temp Indx Mechw/Imp 230° F. UL 746 (0.0580 in) Rel Temp Indx Elect (0.0580 in) 230° F.UL 746

[0074] The shear viscosity data for each polymer were obtained on aRheometrics Mechanical Spectrometer (Model RMS800). A frequency sweepwith five logarithmically spaced points per decade was run from 0.1 to15,000 rad/s at 230C. The strain was determined to be within the linearviscoelastic regime by performing a strain sweep at 0.1 rad/s and 230°C., by strain sweep from 2 to 30 percent strain in 2 percent steps todetermine the minimum required strain to produce torques within thespecification of the transducer; another strain sweep at 100 rad/s and230° C. was used to determine the maximum strain before nonlinearityoccurred according to the procedure disclosed by J. M. Dealy and K. F.Wissbrun, “Melt Rheology and Its Role in Plastics Processing”, VanNostrand, N.Y. (1990). All testing was performed in a nitrogen purge tominimize oxidative degradation. The viscosity data for the PE1 resin andthe PP2 resin are summarized in Table 4 and Table 5, respectively. TABLE4 Dynamic Shear Viscosity data for PE1 Resin Shear Rate (1/sec) ShearViscosity (Poise) 3.54 25274  6 18688  11.07 17110  12.35 16321  25.5612827  39.24 10993  67.62 8790 127.45 6672 206.7 5335 341.79 4135 560.693149 736.41 2595 963.85 2239 1702.31 1537 3122.5  993

[0075] TABLE 5 Dynamic Shear Viscosity data for PP1 Resin Shear Rate(1/sec) Shear Viscosity (Poise) 3.78 80336 6.5 58024 12.22 41594 13.7138761 29.4 24150 46.48 17617 84.21 11680 173.33  6828 312.06  4337614.62  2513 1386.25  1266 2528.58  763 3600.48  551 5734.1  377 9134.55 257 14468.6  174

[0076] The shear viscosity data for the ethylene copolymer and thepolypropylene are plotted as a function of shear rate in FIG. 1. Boththe x axis and the y axis in FIG. 1 are in the logarithmic scale.Referring to FIG. 1, the broken line represents the polypropylene, andthe solid line represents the ethylene copolymer. It is seen that theviscosity curve of the ethylene polymer crosses that of thepolypropylene in the shear rate range from 10 s⁻¹ to 12,000 s⁻¹. Theshear viscosities for the two polymers are substantially “matched” at ashear rate of about 300 s⁻¹.

EXAMPLE 2

[0077] In this example, the shear viscosity of another type ofpolypropylene is compared with that of the PE1 resin. A propylenehomopolymer designated hereinafter as “PP3” was obtained from Montell asKS353P polypropylene with a density of 0.88 g/cc and a melt flow rate of0.45 g/10 min. Additional physical property data for the PP3 resin arelisted in Table 6 below. TABLE 6 Physical Properties of FF3 Resin ASTMMethod KS-353P Typical Resin Properties Melt Flow Rate, g/10 min D 12380.45 Density at 23° C., g/cm³ D 792 0.88 Typical Blown FilmProperties^((a)) Haze, % D 1003 58 Gloss, 45° D 523 9 Water VaporTransmission Rate, E96 (E) 3.1 At 38° C. and 100% RH, g/100 in²/dayTensile Strength at Yield, Machine Direction, psi (MPa) D 882 1,600 (11)Tensile Strength at Yield, Transverse Direction, psi (MPa) D 882 1,000(7) Tensile Strength at Break, Machine Direction, psi (MPa) D 882 3,700(26) Tensile Strength at Break, Transverse Direction, psi (MPa) D 8821,500 10) Tensile Elongation at Yield, Machine Direction, % D 882 46Tensile Elon ation at Yield, Transverse Direction, % D 882 21 TensileElongation at Break, Machine Direction, % D 882 420 Tensile Elongationat Break, Transverse Direction, % D 882 550 Tensile Modulus, 2% Secant,Machine Direction, psi (MPa) D 882 12,000 (85) Tensile Modulus, 2%Secant, Transverse Direction, psi (MPa) D 882 11,000 (78) Elmendorf TearStrength, Machine Direction, g D 1922A 410 Elmendorf Tear Strength,Transverse Direction, g D 1922A 530 Dart Impact, g D 1709A 730

[0078] The shear viscosity of this polypropylene was measured accordingto the procedure described in Example 1. The data obtained for the PP3resin are summarized in Table 7. TABLE 7 Dynamic Shear Viscosity Datafor PP3 Resin Shear Rate (rad/sec) Shear Viscosity (Poise) 2.9823 990237.4557 74802 14.911 52628 29.823 36374 74.557 20353 149.11 12978 298.23 8043 745.57  3820 1491  2088 2982  1142

[0079] The viscosity data for both the PP3 resin and the PE1 resin areplotted as a function of shear rate in FIG. 2. Both x axis and y axis inFIG. 2 are in the logarithmic scale. Referring to FIG. 2, the solid linerepresents the viscosity curve for the ethylene/octene copolymer, andthe broken line represents the viscosity curve for the polypropylene. Ascan be seen, the two viscosity curves do not cross in the shear raterange from 10 to 12,000 s⁻¹.

EXAMPLE 3

[0080] In this example, three types of peelable seals were prepared forcomparison of heat seal performance. Sample 1 was made from 100% PE1resin (the same polymer used in Example 1). Sample 2 was prepared from ablend of 65% PE1 and 35% PP3 (the same as Example 2). The third samplewas made from 65% PE1 and 35% PP1 (the same polypropylene used inExample 1). Peelable seals were made substantially according thefollowing procedure.

[0081] To obtain the desired percentage of polyethylene andpolypropylene, the two polymer components were dry blended by weightpercentage. Then the two components were fed into a 63.5 mm single screwextruder for use as the sealant layer in a three layer co-extruded film.Films were fabricated using an Egan three layer co-extrusion blown filmline. The resin for the sealant layer in a peelable seal was fed to a63.5 mm single flight double mix screw extruder with a 24:1 length todiameter ratio and a melt temperature of 380° to 400° C. The polymerflew through a screen configuration (20/40/60/20 mesh). The extrudedpolymers moved through a transfer pipe into a 20.3 cm (8 inches) diewith a 1.78 mm (70 mil) die gap. The total film thickness was 50 microns(2.0 mils) with the sealant layer accounting for about 20% of thestructure. The two other layers included high density polyethylene. A2.25:1 blow up ratio was used.

[0082] Before analysis, each sample was stored at 25° C. for 24 hours.1″×6″ specimens were positioned lengthwise on a hot tack tester andsealed with a dwell time of 0.2 seconds. The sealed specimens were thenstored for 24 hours. The specimens were then pulled, one at a time, at10 inches per minute to their breaking point on an Instron Tester. Themachine then used the dimensions of the specimen and the force exertedto calculate the seal strength of the specimen. The seal strength wasobtained as a function of seal bar temperature. The data obtained forthe les are summarized in Table 8. TABLE 8 Heat Seal Strength Data 100%PE1 65% PE1/35% PP1 65% PE1/35% PP3 Heat Seal Heat Seal Heat Seal Temp(° C.) Strength (Ib/in) Temp (° C.) Strength (Ib/in) Temp (° C.)Strength (Ib/in) 80 0.70 80 0.79 80 90 2.98 90 2.34 90 0.16 100 5.17 1003.01 100 1.11 110 6.32 110 4.30 110 1.36 120 7.11 120 5.45 120 1.93 1307.42 130 6.67 130 1.88 140 6.82 140 7.29 140 5.00 150 6.72 150 7.39 150—

[0083] The above heat seal strength data are plotted as a function ofseal bar temperature in FIG. 3. The target heat seal strength wasbetween 0.5 and 3 lb/in. Referring to FIG. 3, it can be seen that thepeelable seal obtained from the blend of 65% PE1 resin and 35% PP1 resinhad a relatively constant heat seal strength over a temperature windowfrom about 100° C. to about 130° C. The solid line with symbolsrepresents an imaginary standard peelable seal (the not represent thephysical data from a real polymer blend).

EXAMPLE 4

[0084] In this example, the aging effect of a peelable seal obtained inExample 3 was studied. The peelable seal made from the blend comprising65% PE1 resin and 35% PP1 resin was tested after four weeks. The heatseal strength data was compared with the data obtained from a peelableseal made from the same blend but tested after one day. The data aresummarized in Table 9. TABLE 9 Heat Seal Strength Data for One-DaySample and Four-Week Sample 65% PE1/35% PP1 (One-Day) 65% PE1/35% PP1(4-week) Temp Heat Seal Strength Temp Heat Seal Strength (° C.) (lb/in)(° C.) (lb/in)  90 0.160  90  95  95 0.540 100 1.106 100 0.774 105 1051.192 110 1.364 110 1.036 115 115 1.376 120 1.934 120 1.668 125 1252.750 130 1.878 130 2.162 135 135 4.978 140 4.968 140 6.240

[0085] The data summarized in Table 7 are also plotted in FIG. 4. Thedata indicates that the heat seal strength does not substantially changeover a four week period of time.

EXAMPLE 5

[0086] In this example, a number of sealed bags were produced on astandard vertical form/fill/seal packaging line. Three polymer blendswere used to make the sealant layer of a peelable seal: 70% PE1/30% PP1;64% PE1/36% PP1; and 61% PE1/39% PP1. After the sealed bags were madefrom each blend on a standard vertical form/fill/seal packaging line,the heat seal strength was measured as a function of heat sealingtemperature. The data are summarized in Table 10. TABLE 10 Heat SealStrength Data for Form/Fill/Seal Bags 70% PE1/30% PP1 64% PE1/36% PP161% PE1/39% PP1 Heat Seal Heat Seal Heat Seal Temp (° C.) Strength(Ib/in) Temp (° C.) Strength (Ib/in) Temp (° C.) Strength (Ib/in) 1100.660 110 0.500 110 0.500 115 0.890 115 0.600 115 0.620 120 1.190 1200.600 120 1.030 125 1.930 125 1.300 125 1.090 130 1.800 130 1.790 1301.290 135 2.350 135 1.580 135 1.270 140 3.770 140 2.270 140 3.010

[0087] The heat seal strength data as a function of seal bar temperatureare plotted in FIG. 5. Referring to FIG. 5, it is seen that the heatseal strength substantially falls within the target range of 1-3 lb/in.Moreover, the peelable seal made from the blend of 61% ethylenecopolymer and 39% polypropylene had a substantially constant heat sealstrength over a temperature window from about 110° C. to about 135° C.It was observed that all of the peelable seals in this example gave aclean peel without excessive stringiness. (i.e., no substantial amountof angel hair).

[0088] As demonstrated above, embodiments of the invention provide apeelable seal and various polymers blends for making the peelable seal.The peelable seal made in accordance with embodiments of the inventionmay have one or more of the following advantages. First, the peelableseal has relatively consistent heat seal strength over a relatively widewindow of temperature i.e., from about 100° C. to about 135° C. Thepeelable seal also produces a relatively clean peel, i.e., it does notgenerate excessive stringiness. Although the performance of the peelableseal in accordance with embodiments of the invention is comparable to orbetter than peelable seals made from a blend of ethylene vinyl acetateand polybutylene, the costs are relatively lower. Therefore, acost-effective alternative to existing peelable seals made from anEVA/polybutylene blend is provided. Moreover, the peelable seal made inaccordance with embodiments of the invention is substantially free ofodor, which makes it suitable for many consumer products applications.The peel strength of the peelable seal does not change substantiallyover time. Some peelable seals are relatively easy to open. Otheradvantages and properties are apparent to those skilled in the art.

[0089] While the invention has been described with respect to a limitednumber of embodiments, these embodiments are not intended to limit thescope of the invention as otherwise described and claimed herein.Variations and modifications therefrom exist. For example, the polymerblend for making the peelable seal in embodiments of the invention isnot limited to two polymers. Three or more polymers may be used so longas one polymer forms a continuous phase and other polymers are dispersedin the continuous phase. As an example, a blend of polyethylene,polypropylene, and polybutylene can be used to make a peelable seal. Insome embodiments, the peelable seals or compositions thereof maycomprise additional compounds or components not described herein. Inother embodiments, the peelable seals or compositions thereof do notinclude or are substantially free of any compound or component notenumerated herein. While the target heat seal strength falls between 0.5and 3 lb/in. in various examples, the peelable seal is not so limited tosuch a range for heat seal strength. Instead, a peelable seal may have aheat seal strength between about 0.1 lb./in. to about 20 lb./in. Theappended claims intend to cover all such variations and modifications asfalling within the scope of the invention.

What is claimed is:
 1. A peelable seal, comprising: a mixture of twoimmiscible polymers; a first polymer forming a continuous phase, thefirst polymer having a shear viscosity η₁ at a temperature of about 230°C. and a shear rate of about 100 radian/second; and a second polymerbeing dispersed in the continuous phase, the second polymer having ashear viscosity η₂ at a temperature of about 230° C. and a shear rate ofabout 100 radian/second, wherein the two polymers define a shearviscosity differential: Δ=|(η₁−η₂)/η₁|, and the shear viscositydifferential Δ is less than about 100%.
 2. The peelable seal of claim 1,wherein the peelable seal is a monolayer.
 3. The peelable seal of claim1, wherein the peelable seal includes at least a base layer and a skinlayer, and the skin layer is formed from the mixture of the twoimmiscible polymers.
 4. The peelable seal of claim 1, wherein thepeelable seal includes two base layers and a skin layer, and the skinlayer is formed from the mixture of the two immiscible polymers.
 5. Thepeelable seal of claim 1, wherein the shear viscosity differential Δ isless than about 50%.
 6. The peelable seal of claim 1, wherein the shearviscosity differential Δ is less than about 30%.
 7. The peelable seal ofclaim 1, wherein the shear viscosity differential Δ is less than about20%.
 8. The peelable seal of claim 1, wherein the shear viscositydifferential Δ is less than about 10%.
 9. The peelable seal of claim 1,wherein the shear viscosity differential Δ is less than about 5%. 10.The peelable seal of claim 1, wherein the shear viscosity differential Δis zero or substantially close to zero.
 11. The peelable seal of claim1, wherein the first polymer is an ethylene polymer.
 12. The peelableseal of claim 1, wherein the first polymer is a homogeneoussubstantially linear interpolymer.
 13. The peelable seal of claim 11,wherein the ethylene polymer has a melt index in the range from about0.1 to about 20 g/10 minutes.
 14. The peelable seal of claim 11, whereinthe ethylene polymer has a melt index in the range from about 0.6 toabout 10 g/10 minutes.
 15. The peelable seal of claim 11, wherein theethylene polymer has a melt index in the range from about 1.5 to about 3g/10 minutes.
 16. The peelable seal of claim 1, wherein the secondpolymer is a propylene polymer.
 17. The peelable seal of the claim 16,wherein the propylene polymer has a melt flow rate from about 0.01 toabout 2 g/10 minutes.
 18. The peelable seal of the claim 16, wherein thepropylene polymer has a melt flow rate from about 0.1 to about 1 g/10minutes.
 19. The peelable seal of the claim 16, wherein the propylenepolymer has a melt flow rate from about 0.3 to about 0.6 g/10 minutes.20. The peelable seal of the claim 1, wherein the first polymer is anethylene polymer with a melt index in the range from about 1.5 to about3 g/10 minutes, and the second polymer is a propylene polymer with amelt flow rate from about 0.3 to about 0.6 g/10 minutes.
 21. A peelableseal, comprising a mixture of an ethylene polymer having a melt index inthe range from about 0.1 to about 20 g/10 minutes and a propylenepolymer having a melt flow rate in the range from about 0.01 to about 2g/10 minutes, wherein the ethylene polymer forms a continuous phase ofthe peelable seal, and the propylene polymer is dispersed in thecontinuous phase of the peelable seal.
 22. The peelable seal of claim21, wherein the peelable seal is a monolayer.
 23. The peelable seal ofclaim 21, wherein the peelable seal includes at least a base layer and askin layer, and the skin layer is formed from the mixture of theethylene polymer and the propylene polymer.
 24. The peelable seal ofclaim 21, wherein the peelable seal includes two base layers and a skinlayer, and the skin layer is formed from the mixture of the ethylenepolymer and the propylene polymer.
 25. The peelable seal of claim 21,wherein the propylene polymer is uniformly dispersed in the continuousphase of the ethylene polymer.
 26. The peelable seal of claim 21,wherein the ethylene polymer has a melt index in the range from about0.6 to about 10 g/10 minutes.
 27. The peelable seal of claim 21, whereinthe ethylene polymer has a melt index in the range from about 1.5 toabout 3 g/10 minutes.
 28. The peelable seal of claim 21, wherein theethylene polymer has a density from about 0.86 g/cc to about 0.97 g/cc.29. The peelable seal of claim 21, wherein the ethylene polymer has adensity from about 0.86 g/cc to about 0.92 g/cc.
 30. The peelable sealof claim 21, wherein the ethylene polymer has a density from about 0.88g/cc to about 0.92 g/cc.
 31. The peelable seal of claim 21, wherein theethylene polymer and the propylene polymer define a shear viscositydifferential: Δ=|(η_(e)−η_(p))/η_(e)|, in which η_(e) and η_(p) are therespective shear viscosity of the ethylene polymer and the propylenepolymer at a shear rate of about 100 radian/second and a temperature ofabout 230 ° C., and the shear viscosity differential Δ is less than100%.
 32. The peelable seal of claim 31, wherein the shear viscositydifferential Δ is less than 50%.
 33. The peelable seal of claim 31,wherein the shear viscosity differential Δ is less than 25%.
 34. Thepeelable seal of claim 31, wherein the shear viscosity differential Δ isless than 10%.
 35. The peelable seal of claim 31, wherein the shearviscosity differential Δ is less than 5%.
 36. The peelable seal of claim31, wherein the shear viscosity differential Δ is zero or substantiallyzero.
 37. The peelable seal of claim 21, wherein the ethylene polymer isa homopolymer.
 38. The peelable seal of claim 21, wherein the ethylenepolymer is a copolymer or interpolymer.
 39. The peelable seal of claim21, wherein the ethylene polymer is selected from the group consistingof high density polyethylene, low density polyethylene, linear lowdensity polyethylene, very low density polyethylene, and ultra lowdensity polyethylene.
 40. The peelable seal of claim 21, wherein theethylene polymer is produced by a metallocene catalyst.
 41. The peelableseal of claim 21, wherein the ethylene polymer is produced by a singlesite catalyst.
 42. The peelable seal of claim 21, wherein the ethylenepolymer is produced by a constrained geometry catalyst.
 43. The peelableseal of claim 21, wherein the ethylene polymer is a homogeneoussubstantially linear ethylene polymer.
 44. The peelable seal of claim21, wherein the ethylene polymer has long chain branching.
 45. Thepeelable seal of claim 21, wherein the propylene polymer is ahomopolymer.
 46. The peelable seal of claim 21, wherein the propylenepolymer is a copolymer or interpolymer.
 47. The peelable seal of claim21, wherein the propylene polymer has a melt flow rate in the range fromabout 0.01 to about 2 g/10 minutes.
 48. The peelable seal of claim 21,wherein the propylene polymer has a melt flow rate in the range fromabout 0.1 to about 1 g/10 minutes.
 49. The peelable seal of claim 21,wherein the propylene polymer has a melt flow rate in the range fromabout 0.3 to about 0.5 g/10 minutes.
 50. The peelable seal of claim 21,wherein the ethylene polymer has a melt index in the range from about1.5 to about 3 g/10 minutes, and the propylene polymer has a melt flowrate of less than about 0.6 g/10 minutes.
 51. A method of making apeelable seal, comprising: obtaining an ethylene polymer having a meltindex in the range from about 0.1 to about 20 g/10 minutes; obtaining apropylene polymer having a melt flow rate in the range from about 0.01to about 2 g/10 minutes; mixing the ethylene polymer and the propylenepolymer to obtain a blend therefrom; and forming a peelable seal fromthe blend, wherein the ethylene polymer forms a continuous phase in thepeelable seal, and the propylene polymer forms a dispersed phase in thepeelable seal.
 52. A method of making a peelable seal, comprising:obtaining a first polymer having a shear viscosity η₁ at a temperatureof about 230° C. and a shear rate of about 100 radian/second; obtaininga second polymer having a shear viscosity η₂ at a temperature of about230° C. and a shear rate of about 100 radian/second, the two polymersdefining a shear viscosity differential Δ=|(η₁−η₂)/η₁| less than 100%;mixing the first polymer and the second polymer to form a blend; andforming a peelable seal from the blend, wherein the first polymer formsa continuous phase in the peelable seal, and the second polymer forms adispersed phase in the peelable seal.
 53. A polymer blend compositionfor a peelable seal, comprising: a blend of an ethylene polymer having amelt index in the range from about 0.1 to about 20 g/10 minutes and apropylene polymer having a melt flow rate in the range from about 0.01to about 2 g/10 minutes, wherein the ethylene polymer is capable offorming a continuous phase of the peelable seal, and the propylenepolymer is capable of being dispersed in the continuous phase of thepeelable seal.
 54. A polymer blend composition for a peelable seal,comprising: a blend of two immiscible polymers; a first polymer capableof forming a continuous phase, the first polymer having a shearviscosity η₁ at a temperature of about 230° C. and a shear rate of about100 radian/second; and a second polymer capable of being dispersed inthe continuous phase, the second polymer having a shear viscosity η₂ ata temperature of about 230° C. and a shear rate of about 100radian/second, wherein the two polymers define a shear viscositydifferential: Δ=|(η₁−η₂)/η₁|, and the shear viscosity differential Δ isless than about 100%.